Metal detector circuits find wide application in locating hidden metal objects, particularly when such objects are located just beneath the surface of the ground. A basic technique for detecting such objects is the use of electromagnetic induction. An electromagnetic field is created in a given region where a hidden object may be located. When a metal object is encountered in this region, the field is altered. The change in the field is indicated in a receive signal derived from a coil located in the electromagnetic field. One problem with this method arises when the target object is located within soil that contains ferrous material. Iron oxide is a typical ferrous material and is very widely distributed on the surface of the earth. It is very common to encounter ferrous material in the process of searching for hidden objects. The ferrous material produces a change in the receive signal which can mask the target signal or produce a false indication of a target.
In many locations a desired object may be located among scattered undesired objects which can produce a response to an electromagnetic induction metal detector. Such undesired objects include pull tabs, bottle caps, aluminum foil, nails and small pieces of iron. Since the unwanted objects typically outnumber the desired objects, it is necessary to provide discrimination between the various types of objects which may be located in the desired region to be searched. It has been discovered that different types of materials produce different phase responses for the receive signal. These different phase responses can be used as a means to discriminate between various types of objects so that there is a greater probability that the desired type of object will be located rather than a piece of junk, such as a bottle cap. Such phase discrimination circuits are shown, for example, in U.S. Pat. Nos. 4,128,803, 3,676,772 and 4,303,879. However, the circuits heretofore used have operated on a principle of discriminating on the basis of a particular phase angle in the receive signal. Since by its very nature in searching for unknown objects, the operator does not know what may be in the search region. He may possibly select a circuit setting that may not respond to a particular type of desired object or may not reject a particular type of unwanted object. Conventional phase discrimination circuits can be helpful but likewise can serve to mislead the operator when an indication of a target is received.
In view of the problems regarding the detection of metal objects by use of electromagnetic induction, there exists a need for an improved metal detector circuit and method for detecting metal objects, particularly in a background of ferrous soil and unwanted metal, by having a selectable range of phase responses for acceptance and rejection.